Sign for one going north by car (6)
I believe the answer is:
gemini
'sign' is the definition.
(Gemini is a star sign)
'for one going north by car' is the wordplay.
'for one' becomes 'eg' (both can mean 'for example').
'going north' shows that the letters should be reversed in order.
'by' means one lot of letters go next to another.
'car' becomes 'mini' (Mini Cooper).
'eg' reversed gives 'ge'.
'ge'+'mini'='GEMINI'
(Other definitions for gemini that I've seen before include "Twins star-sign" , "Third sign of the Zodiac" , "Twins star sign" , "star group" , "space programme" .)